Single level spine fusion procedure typically entails removing the intervertebral disk (not shown) and inserting an interbody device 2 into the disk space 4, as shown in FIG. 1.
Current spine fusion procedures rely heavily on the use of posterior fixation to achieve the stability and rigidity necessary to obtain successful clinical results. However, implantation of posterior instrumentation necessarily involves removing important musculoskeletal elements.
Because of these concerns, anterior fixation systems have also been developed which require removal of much less musculoskeletal mass. However, because anterior fixation, especially in the abdominal or thoracic area, lies in close proximity to vital internal organs such as the aorta, these fixation systems must also possess a low profile.
In general, conventional intervertebral connection systems can be characterized by ligament components which are either a) relatively rigid, or b) not shaped for cooperative connection to bone fasteners, or by bone fasteners which are shouldered to seat upon the vertebral surface. When the ligament is relatively rigid, it must essentially fully lie upon the anterior surfaces of the adjacent vertebrae, thereby limiting design options. Systems having relatively rigid ligaments typically have transverse holes near their end portions for accepting bone fasteners. In systems in which the ligament is not shaped for cooperative attachment to the bone fastener, attachment is typically made by either suturing or by passing a screw through the ligament. When the bone fastener is seated upon the vertebral surface, a portion of the bone fastener protrudes from the surface and the tension of the ligament can not be further adjusted.
U.S. Pat. No. 5,415,661 (“Holmes”) discloses an implantable spinal assist device apparently intended for posterior use comprising a ligament 4 having opposed terminal ends 6. The opposed terminal ends have transverse openings for accepting a pair of undescribed conventional bone screws. Although the ligament can be “fully compliant”, it receives its flexibility by designing a curve into central portion 10 and intermediate portions 12. Accordingly, the terminal portions 8a, 8b of the ligament are relatively straight and stiff and are preferably made of fiber/polymer composites. Since these terminal portions 8a, 8b are relatively rigid, the ligament must essentially fully lie upon the anterior surfaces of the adjacent vertebrae, thereby limiting design options.
In addition, if the Holmes device were used for anterior fixation, its lack of control of the bone screw profile could cause the screw head to protrude towards the aorta. Further, there is no teaching in Holmes that the ligament or screws could be bioresorbable.
U.S. Pat. No. 6,136,001 (“Michelson”) discloses a spinal implant having an integral staple member 12 comprising top member 14 having projections 16, 17 at either end thereof for insertion into the adjacent vertebrae. There is no disclosure in Michelson that the any part of this staple member should be flexible. Accordingly, this staple does not provide the degree of flexibility provided by a natural ligament, and so is prone to failure during compression of the spine. Such a failure could spread device fragments about the area of the aorta and lungs. The rigidity of the staple also hinders the loading of the interbody fusion cage, thereby producing undesirable stress shielding. The rigidity of this staple also makes it unsuitable for use with intradiscal devices designed to provide a measure of motion (e.g., motion discs).
In addition, the Michelson staple has a thickness in the range of 2.0 mm to 4.0 mm and can be bioresorbable.
PCT Patent Publication No. WO 00/59388 (“Middleton”) discloses an artificial spine ligament 100 comprising a conformable plate 102 having transverse longitudinal holes for accepting bone screws 114. Although Middleton teaches that the ligament should be conformable, the conformability feature appears to relate to the plate's ability to conform to the surface of the vertebral bodies, not to bend around corners. Accordingly, the preferred embodiment discloses an apparently monolithic polyethylene uniplanar-plate having transverse holes for accepting screws. These screws have shoulders which seat upon a countersunk portion of the transverse holes. In addition, since the five-piece assembly of Middleton is not integral, it allows for micromotion between the various components. Middleton discloses that the heads of the screws should be flush with the top surface of the plate. Middleton further discloses that the ability of the plate to accept tensile or compressive loads can be adjusted by changing the location of the screw in the transverse longitudinal holes. Lastly, there is no disclosure in Middleton of a bioresorbable component.
U.S. Pat. No. 5,180,393 (“Commarmond”) discloses an artificial spinal ligament made of a flexible textile material having hollow eyelets at each end. Each eyelet is shaped to receive the head of a fixation screw. The ligament comprises a secondary winding which gives the ligament stiffness during compression, thereby providing stability to the system. The stiffness of this ligament would likely cause it stress if it were to be bent around a corner. Accordingly, the preferred embodiment discloses uniplanar ligaments. The eyelets of the Commarmond device include concave seats for seating screws. Once these screws are seated, the tension on the ligament can not be adjusted.
Respecting profile, Commarmond discloses dimensions of the ligament in the 5-10 mm range. In addition, the use of a conventional screw causes the device to protrude somewhat from the vertebral surfaces, and so it would be unsuitable for use in anterior applications. Respecting resorbability, Commarmond discloses a non-resorbable polyester ligament, and metal eyelets and screws.
U.S. Pat. No. 5,865,846 (“Bryan”) discloses a motion disc-type of spinal prosthesis having a strap-like ligament 250 made of Kevlar-like and/or Goretex-like materials, which is attached to the vertebral bodies by screws 94. The preferred embodiment discloses a uniplanar strap attached to the vertebral bodies by transverse screws having shoulders which seat upon the strap.
Although Bryan discloses bioresorbable screws, since the ligament is made of Kevlar-like and/or Goretex-like materials, it is non-resorbing and so will permanently remain in the body.
A few references disclose flexible ligaments whose ends are not shaped for cooperative connection to bone fasteners. For example, U.S. Pat. No. 6,093,205 (“McLeod”) discloses a spinal implant having a fabric element 40 which is secured across adjacent vertebrae by passing sutures therethrough to undescribed bone fasteners implanted into the adjacent vertebrae. The use of sutures as a ligament-bone fastener connection means is disadvantageous for many reasons. For example, because sutures are typically weak, they are prone to failure, thereby risking detachment of one end of the ligament in the vicinity of the aorta. They produce localized stresses in the portions of the ligament and bone fastener to which they attach. Their resorption time is often far too quick to be suitable for use as part of a vertebral connection system in spinal fusion. Lastly, their fixation must occur intraoperatively, thereby increasing the duration and difficulty of the operation. Although fabric element 40 may be made of bioresorbable materials, there is no disclosure in McLeod that the bone fastener should be bioresorbable.
U.S. Pat. No. 5,681,310 (“Yuan”) discloses a spinal fixation device comprising a flexible mat 10 fastened across the invertebral space by fastening elements 20. Although Yuan discusses bioabsorption of the device components, the materials disclosed therein as those suitable for use as components, such as DACRON and metals, do not bioabsorb within about 24 months and so do not accommodate growth of the patient. Thus, this system may not be desirable for applications in which the patient may not yet be fully-grown. Further, FIG. 5 of Yuan discloses a fastening screw 20″ having a fastening cord 25 attached thereto, wherein the screw 20″ is inserted through the mat 10 and into the vertebra, and then is secured by the fastening cord. A portion of the mat 10 is thus destroyed during the attachment process. Further, Yuan does not disclose any means for increasing tension upon the fastened ligament. Lastly, the intraoperative attachment of the mat to the screw increases both the duration and difficulty of the operation.
Many generic ligament repair devices have been described. For example, PCT Patent Publication No. WO 91/06249 (“Collins”) discloses a prosthetic ligament attached to the bone via staples. PCT Patent Publication No. WO 00/72782 (“Wolowacz”) discloses a flexible elongate tape fixed to the bone by either staples, bone fasteners, or screw-washer combinations, optionally in combination with a “figure of eight” eyelet loop at the end of the device.
Thus there is a need for an intervertebral connection system which can easily connect a compliant ligament to a bone fastener without significantly straining the ligament, without suturing, and without destroying a portion of the ligament.